The electrophoretic effect is a well known and the prior art is replete with a number of patents and articles which describe the effect. As will be recognized by a person skilled in the art, the electrophoretic effect operates on the principle that certain particles, when suspended in an medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPID) utilize the electrophoretic effect to produce desired images. In prior art EPID colored dielectric particles are suspended in a fluid medium that is either clear or of an optically contrasting color as compared to the dielectric particles. The colored electrophoretic particles are then caused to selectively migrate to, and impinge upon, a transparent screen, thereby displacing the fluid medium against the screen and creating the desired image.
As will be recognized by a person skill in the art, the selection of the electrophoretic particles skilled in the EPID is very important in determining the performance of the EPID and the quality of the viewed image produced. Ideally, electrophoretic particles should all be of a uniform size, to help in assuring that each of the electrophoretic particles will behave similarly. Additionally, it is desirable to utilize electrophoretic particles that have essentially the same density as the fluid medium in which that are suspended. By using electrophoretic particles of essentially the same density as the suspension medium, the migration of the electrophoretic particles through the medium remains independent of both the orientation of the EPID and the forces of gravity.
To affect the greatest optical contrast between electrophoretic particles and the suspension medium, it is desirable to have either white particles suspended in a black medium or black particles suspended in a backlighted clear medium. In the prior art, it has proven difficult to produce black electrophoretic particles that are dielectric, of uniform size, and which have a density matching that of a common suspension medium. As a result, EPIDs commonly use readily manufactured light colored electrophoretic particles suspended in dark mediums. Such EPIDs are exemplified in U.S. Pat. Nos. 4,655,897 to DiSanto et al., 4,093,534 to Carter et al., 4,298,448 to Muller et al., and 4,285,801 to Chaing. In such prior art where light particles are suspended in a dark medium, the suspension often appears grayish until the application of an electric field. With the electric field applied, the light colored particles migrate through the grayish suspension producing a light image on a gray background, thereby resulting an image that is not highly contrasted.
To produce a more contrasted image, it is desirable to backlight suspended black particles in a clear medium. However, as has been mentioned, the development of suitable dielectric black particles remains a long felt need in the art of electrophoretic image displays. In arts other than EPIDs, black particles are commonly produced from carbon. However, carbon blacks are not readily adaptable to EPIDs because carbon blacks are conductive and the density of carbon blacks is not readily matched to a suitable suspension medium. Research efforts have been made in an attempt to solve the density and conductivity problems of carbon blacks, however, none has succeeded without trading off the blackness (i.e. energy absorbency) of the particles created. Such efforts to produce dielectric particles from carbon blacks are exemplified in the following articles: Fowkes et al. "Electrophoretic Display Medium", a research project report for the Department of Chemistry at Lehigh University (Aug. 28, 1989) and Hou et al. "Polymer-Encapsulated Particles With Controlled Morphologies." PH.D Dissertation, (Lehigh University, 1991).
The present invention does not use carbon black as the source of the electrophoretic particle. Rather, composite latexes stained with a metal oxide are used to form the dielectric black particles suitable for use in a EPID. More particularly, the preferred embodiment of the present invention produces black particles from seeded emulsion polymerization techniques, used to produce core/shell latex structures with the residual double bonds, that are stained black with a metal oxide. The density, blackness, particle size and surface characteristics of the present invention black particle are controlled by the polymer composition, crosslink density, residual double bond density and reaction conditions.
The development of particles from synthesize core/shell latexes has been addressed in numerous technical articles, as exemplified by the following: Wessling et al. "A Study Of Emulsion Polymerization Kinetics by the Method of Continuous Monomer Addition." Journal of Macromolecular Science., A7 (3), pp. 647-676 (1973); Keusch et al. "The Growth of Polystyrene Latex Particles." Journal of Macromolecular Science., A7 (3) pp. 623-646 (1973); Grancio et al. "The Morphology of the Monomer-Polymer Particle in Styrene Emulsion Polymerization." Journal of Polymer Science., vol. 8, pp 2617-2629 (1970); Grancio et al. "Molecular Weight Development in Contrast-Rate Styrene Emulsion Polymerization." Journal of Polymer Science., vol. 8, pp. 2733-2745 (1970); and Wiener, H. "Polymerization in the System Vinylidene Chloride-Potassium Laurate-Potassium Persulfate", Journal of Polymer Science vol. 7, pp. 1-20 (1951). Of more direct relation to the present invention process of producing black particles are the below referenced articles.
In Daniels et al. "Preparation of Acrylonitrile/Butadiene/Styrene Latexes Using Hydroperoxide Redox Initiators." Journal of Applied Polymer Science, vol. 41, pp. 2463-2477 (1990), an acrylonitrile/butadiene/styrene composite latex is shown where the polybutadiene core is uniformly surrounded by a poly(styrene-co-acrylonitrile) shell. In Daniels, batch and semi-continuous seeded emulsion polymerization techniques are used with varying core and shell ratios and other reaction parameters.
In Merkel, M. P. "Morphology of Core/Shell Latexes and their Mechanical Properties" PH.D. Dissertation (Lehigh University 1986), seeded emulsion polymerization is utilized to synthesis polybutadiene-poly(methyl methacrylate) core/shell latexes. The core/shell latexes have various levels of crosslink density of the core and various thickness of the shell.
In Sundberg et al. Journal of Dispersion Science Technology vol. 5, pp. 433 (1984), synthesized polybutadiene-polystyrene core-shell latexes are studied in various conditions, such as monomer/polymer ratio, initiator level, degree of conversion and concentration of chain transfer agent, to determine grafting efficiencies of styrene onto polybutadiene latexes.
The use of staining agents on polymers was first used to form contrasts helpful in viewing polymer structures through electron microscopy. The prior art pertaining to such polymer staining is exemplified in the following articles. In Gaylarde et al. Science, vol 161, pg. 1157 (1968) ruthenium tetraoxide was used as a staining agent for polymeric materials in electron microscopy. In Vitali et al. Polymer, vol 21, pg. 1220 (1980) ruthenium tetraoxide was used to improve image contrast for polybutadiene lattices, a terpolymer of acrylnitride, butadiene and styrene, and an acrylnitrile-styrene-acrylnitrile polymer. In Trent et al. Journal of Polymer Science, volume 19, pg, 315 (1981), ruthenium tetroxide was used in vapor staining polystyrene/poly(methylmethacrylate) blends for electron microscopy studies. Finally, in Trent et al. Macromolecules, vol 16, pg. 589 (1983) ruthenium tetraoxide was shown having the capability of staining both saturated and unsaturated polymer systems that contain ether, arene, alcohol, aromatic, amide, or olefin moieties.
However, none of the above referenced prior art addresses a process of producing dielectric particles having a high degree of blackness with a controlled density and particle sizes so as to be adaptable to an electrophoretic image display. It is, therefore, an object of the present invention to provide an improved electrophoretic particle that has a high degree of blackness, a controlled particle size, surface functionality and density so as to be readily suspended in the liquid medium of an electrophoretic image display.